An important component of disposable absorbent articles, such as diapers, is an absorbent core structure comprising water-absorbing polymers, e.g. hydrogel-forming water-swellable polymers, also referred to as absorbent gelling material (AGM), or super-absorbent polymers (SAP's). This polymer material ensures that large amounts of bodily fluids, e.g. urine, can be absorbed by the article during its use and locked away, thus providing low rewet and good skin dryness.
Especially useful water-swellable polymers or SAP's are often made by initially polymerizing unsaturated carboxylic acids or derivatives thereof, such as acrylic acid, alkali metal (e.g., sodium and/or potassium) or ammonium salts of acrylic acid, alkyl acrylates, and the like in the presence of cross-linking compounds, such as (relatively small amounts of) di- or poly-functional monomers such as N,N′-methylenebisacrylamide, trimethylolpropane triacrylate, ethylene glycol di(meth)acrylate, or triallylamine. The di- or poly-functional monomer materials serve to cross-link the polymer chains thereby rendering them water-insoluble, yet water-swellable. These cross-linked absorbent polymers contain a multiplicity of carboxylate groups attached to the polymer backbone. It is generally believed, that the neutralized carboxylate groups generate an osmotic driving force for the absorption of body fluids by the cross-linked polymer network.
Such reactions are typically done with aqueous solutions of the monomers, resulting cross-linked polymers that have absorbed water (they are typically swollen hydrogels). The water is then subsequently removed from these materials, to render dry particulate water-absorbing (swellable) polymers. The removal of water is an energy-consuming process. Hence, there is a desire to provide less energy consuming processes.
One proposed route would be to use less water during the production of the water-absorbing cross-linked polymers. However, the inventors found that when highly concentrated monomer solutions are used, the resulting polymers may be more physically entangled, which can negatively impact the performance. Additionally, the reaction often is so exothermic that high monomer concentrations could lead to explosions.
Furthermore, and in contrast, the inventors found that it may be beneficial to use less concentrated monomer solutions, to obtain less entangled (e.g. more linear polymers) water-absorbing cross-linked polymers. However, this would require an even more energy intensive drying process.
The inventors have now found a method for producing water-absorbing cross-linked polymers, such as cross-linked polyacrylic acids/polyacrylates using a supercritical medium, including or consisting of, for example, supercritical carbon dioxide, that can be easily removed, e.g. by pressure reduction. This method can reduce the energy consumption associated with the removal of large amounts of water, and it can provide water-absorbing cross-linked polymers, e.g. in particulate form, that can even be made porous, to improve the rate of absorption of fluids, such as urine and blood, and that can be effectively used in absorbent articles.